Silver halide photographic emulsions are conventionally prepared by reacting an aqueous alkali metal halide or ammonium halide solution with an aqueous silver salt solution, e.g. silver nitrate, in the presence of a protective colloid, e.g. gelatin, to cause precipitation of small silver halide nuclei. After physical ripening to the desired average grain size, the emulsion is subjected to chemical ripening.
At the conclusion of the physical ripening, the photographic emulsions contain soluble by-products and excess halides which should be removed.
The soluble by-products and excess halides can be removed by chill-setting the emulsion, comminuting it into small fragments and washing by suspending in water, as illustrated in U.S. Pat. Nos. 2,316,845 and 3,396,027. Other methods include coagulation washing as illustrated in U.S. Pat. Nos. 2,618,556, 2,614,928, 2,565,418, 3,241,969, 2,489,341, and in GB Pat. Nos. 1,305,409 and 1,167,159; centrifugation and decantation of the coagulated emulsion as illustrated in U.S. Pat. Nos. 2,463,794, 3,707,378, 2,996,278 and 3,489,454; employment of hydrocyclones alone or in combination with centrifuges as illustrated in GB Pat. Nos. 1,336,692 and 1,356,573; diafiltration with a semipermeable membrane as illustrated by Research Disclosure 10208, October 1972, Research Disclosure 13122, March 1975, DE Pat. Appl. No. 2,436,461, U.S. Pat. No. 2,495,918, or with an ion-exchange resin as illustrated by U.S. Pat. Nos. 3,782,953 and 2,827,428.
Preferred methods of emulsion washing are those based on coagulation washing. Coagulation washing is based on the principle of causing the gelatin to coagulate so that it settles together with the silver halide grains and thus separates from the aqueous solution comprising the soluble by-products. The supernatant solution is decanted and the settled gelatin-silver halide complex is redispersed, after washing of the precipitate, in water or aqueous gelatin. The coagulation washing method offers a number of advantages; in particular, a high percentage of unwanted by-products can be removed in one stage, concentrated silver halide emulsions can be prepared, and the process is simple and inexpensive. Coagulation is conventionally effected by addition of inorganic electrolites, such as ammonium, potassium and sodium sulfate, as described in U.S. Pat. No. 2,618,556; acid-coagulable gelatin derivatives, as described in U.S. Pat. Nos. 2,494,041, 2,614,928, 2,614,929, 2,728,662, 2,768,079, 2,787,545, 2,956,880, 3,118,766, 3,132,945, 3,138,461, 3,359,110 and 3,867,154; anionic soaps, as described in U.S. Pat. Nos. 2,489,341 and 2,527,260; polymeric compounds containing carboxyl groups, as described in U.S. Pat. No. 2,565,418; polymeric compounds containing sulfo groups, as described in U.S. Pat. No. 3,137,576; and polymeric compounds containing both carboxyl and sulfo groups, as described in U.S. Pat. Nos. 3,168,403 and 3,241,969. It is also known to combine different coagulating agents in coagulation washing of silver halide emulsions; for example, GB 892,464 discloses the combination of the cleavage product of a keratin-containing substance and heavy metal salts, U.S. Pat. No. 3,436,220 discloses the combination of polyoxyalkylenated derivatives of gelatin and polystyrene sulfonic acid or ammonium sulfate, and U.S. Pat. No. 3,884,701 discloses the combination of an acid-coagulable gelatin derivative and a low molecular weight polystyrene sulfonic acid.
The importance of coagulation washing method is illustrated by the large selections of variants of the method which have been suggested. However, the coagulation washing methods so far proposed have been found to suffer from one or more disadvantages which greatly reduce their over-all effectiveness. For example, considerable amounts of time are consumed while waiting for the coagulate to settle, particularly when the coagulate is redispersed and reprecipitated to repeat the washing step more than once, as is ordinarily done. In addition, such methods present considerable difficulty in connection with washing operations which usually require considerable amounts of water and time. Moreover, there are difficulties in using known coagulation washing methods for the coagulation of very fine grain silver halide emulsions, such as those used in Graphic Arts films, in that undesired sedimentation and agglomeration of silver halide grains may occur.
Ionic copolymers of styrene sulfonic acid and maleic acid are disclosed in auxiliary antistatic layers of silver halide photographic elements, as described, for example, in U.S. Pat. Nos. 4,460,679, 4,585,730, 4,891,308, 4,960,687 and in EP 391,402 and 391,176. Ionic copolymers of styrene sulfonic acid and maleic acid are also disclosed in U.S. Pat. No. 4,391,903 to improve covering power when added to gelatin binder employed in medical silver halide X-ray emulsions.